The present invention relates generally to biological tissue stimulators and more particularly to biological tissue stimulators utilizing a constant current output stage.
Biological tissue stimulators are known to be medically useful. In one example, a transcutaneous electrical nerve stimulator (TENS) is utilized to mask pain signals in a human body before they reach the brain, giving the subject apparent relief from the pain. In such a TENS device, electrical stimulation pulses, usually current pulses of a selected rate, amplitude, pulse width and duty cycle, are delivered to the skin of the subject by one or more electrodes, each containing a pair of electrode elements. The timing characteristics of the delivered electrical stimulation pulses may be predetermined, as for example, by the prescribing physician and/or may be individually selected or controlled by switches available to be operated by the subject. Additionally, individual parameters, or even entire pulse programs, can be varied in a predetermined or random basis by the TENS device itself.
Another example of a useful biological tissue stimulator is a neuromuscular stimulator (NMS) which can be utilized to electrically stimulate muscle activity of a patient. Electrical stimulation pulses, again probably current pulses of a carefully controlled rate, amplitude, pulse width and sequence are delivered by one or more pairs of electrodes to a site or sites near the muscle to be stimulated in order to activate or contract the muscle. The initiation and control of such sequence of pulses may be patient-controlled.
Biological tissue stimulators typically provide a series of electrical stimulation pulses to the biological tissue according to a preset program. It is generally desirable to provide an electrical stimulation pulse of a known current value. Output circuits are commonly arranged to produce so-called "constant current" outputs. The output stage of a biological tissue stimulator sees a load impedance which consists primarily of the electrode-tissue interface impedance and the impedance of the biological tissue between the electrode elements. This impedance can vary significantly, for example, from about 200 ohms to 2500 ohms. The "constant current" output stage supplies the same current to the worst-case expected load impedance, i.e., the highest expected load impedance, as to a more normal (lower) load impedance. The high voltage power supply driving the output stage must be able to deliver a high enough voltage to maintain the desired current level through this "worst-case" load impedance. If the high voltage power supply output level is fixed at this maximum value, however, then a much higher voltage is supplied to the output stage than is otherwise required when the load impedance is in a lower, more normally anticipated, range. This excess high voltage power is then dissipated and "lost", resulting in an energy-inefficient use of such a high voltage supply.
It is highly desirable that biological tissue stimulators be portable devices, that is, be compact in size and low in weight Both attributes make for a more comfortable and easily used biological tissue stimulator. The biological tissue stimulator may be required to be connected to the body for extended periods of time and, thus, ease of portability and unobtrusivness are of critical importance.
For portability, biological tissue stimulators must be battery-powered since connection to a household power supply would greatly limit the geographical range of operation. It is advantageous that the battery life of the biological tissue stimulator be as long as possible. Insufficient battery life limits the freedom of extensive use without resupply, creates the bothersome duty of physically changing the batteries and substantially increases the cost of operation. Also, the deterioration of battery voltage may result in less than optimum performance characteristics. If the battery discharges more often, a greater percentage of time may be spent during nonoptimum operating conditions and, thus, the biological tissue stimulator may not achieve the advantageous results that it was intended to produce.
Battery life may be increased by increasing the size and weight of the batteries. This, however, is not desirable since increasing of the size and weight of the biological tissue stimulator limits the places, locations and environments for the biological tissue stimulator is likely to be used. Thus, a heavy, bulky biological stimulator is not truly portable.